In the past, when architectural lighting designers were faced with the task of uniformly illuminating the playing surface of a sports field while minimizing the amount of light spilled into the seating portion of the sports field, the individual lighting fixtures at their disposal would be of a type that utilized such light delivery techniques as glare shields and louvers. The aim of such a designer is to substantially reduce or even eliminate the amount of light that extends above the playing surface and into the eyes of the spectators and even above the confines of the field so as to adversely affect the surrounding homes and/or businesses. It is known that though such glare shields and louvers can be effective to direct light output in a desired pattern, in doing so, these devices contribute a measurable amount of light loss that therefore requires an increased amount of light output from the light source. In addition to resulting in light loss, a typical sports lighting fixture provides such shield and louver devices in an external manner to the reflector portion so as to adversely contribute to the wind loading characteristics of such light fixture. An example of a light fixture having external glare shield and louver elements is found in U.S. Pat. No. 4,725,934 issued to Gordon on Feb. 16, 1988. Given that each louver results in a specific amount of light loss and that external light directing elements increase wind loading of the fixture, it would therefore be advantageous that if further light directing elements were needed to direct light output so as to minimize spill light above the necessary playing surface, that the number of such devices be minimized and that they reside within the reflector configuration rather than on the outside thereof.
The number of louvers that are needed to redirect light output is directly related to how large of an upper light output angle there is relative to the longitudinal axis of the reflector of the fixture. The larger the angle of light output from the reflector, the greater the amount of louver area is needed to redirect such light to the specific location desired. As seen it the previously cited U.S. Pat. No. 4,725,934, the reflector portion is essentially elliptical in shape and as such results in a large output angle of light as it exits the reflector. It should also be noted that in such fixture, the light source is disposed on the longitudinal axis of the reflector thereby resulting in light being generated near the outer edge of the reflector. The disposition of the light source in this manner is required because of the size of the outer jacket of the light source, however, it is known that such an orientation of the light source relative to the reflector results in a large angle of light output. It would therefore be advantageous if a light source, reflector orientation could be provided that resulted in as collimated a light output from the reflector as possible so as to achieve a minimum spread of light output falling at large angles relative to the longitudinal axis of the reflector.
Toward the goal of maximizing the amount of light output from an individual reflector that can be utilized on the playing surface, it is also known that the reflector portion of the fixture is typically constructed of aluminum which is polished so as to achieve a high reflectivity value. The use of aluminum reflectors is well known for sports lighting fixtures. One of the disadvantages however with an aluminum reflector is the fact that the reflectivity is such that only about 75-85% of the light generated by the light source is actually reflected by such aluminum reflector out of the fixture. Additionally, if a bare quartz arc tube light source is utilized in conjunction with an aluminum or other metallic reflector in a manner to be in close proximity to the metallic reflector, it has been observed that photons emitted by the light source strike the reflector and result in electrons bouncing back at the quartz material which has the effect of degrading the quartz material of the arc tube. If a light source is rated having a life of over 3000 hours, such degradation of the arc tube material will be sufficient to cause early failure of the light source. Accordingly, it would be further advantageous if a reflector, light source arrangement could be achieved which would have a reflectivity value of about 95% and would not cause the degradation of the quartz arc tube of the light source. As an example of a commercially available sports lighting fixture using an aluminum reflector and a bare quartz arc tube light source can be found in the product designated "ArenaVision" which is available from Philips Lighting.
Yet another problem encountered by the lighting designer of a sports field is the determination of the optimum combination of fixtures and the tilt of such fixtures so as to achieve as efficient and uniform of a pattern of illumination as possible. More particularly, since the light pattern at the playing field surface is actually made up of a combination of a number of light output patterns from the various lighting fixtures located along the periphery of the sports field, it is necessary to combine such light outputs in a manner to minimize the number of fixtures needed for the task. In conventional practice, the light output pattern of an individual reflector is circular so that, once it is projected downward and at an angle from the mast structure on which the fixture is mounted, the light pattern at the playing field surface is essentially elliptical with the highest intensity of light output occurring at the foci closest to the fixture. For a discussion of the development of the above referenced ArenaVision sports lighting fixture, reference is made to the paper delivered at the June 1988 IES Annual Conference by Entrop and Verbeek of Philips Lighting entitled "System Oriented Development of Sports Lighting Equipment to Cover New Trends". In this paper, there is significant discussion of the use of a discharge light source which achieves an essentially point source arc configuration. By such reliance on a point source of light, it can be seen that the ArenaVision sports lighting fixture will generate a circular light pattern as measured in a conventional photometric manner thus resulting in the essentially elliptically shaped pattern at the playing field surface. In puzzling together a combination of such elliptical shaped lighting patterns, the lighting designer invariably experiences an inefficient overlapping configuration of the light output at the field surface and will also experience spill light which extends beyond the playing field surface and into the eyes of the spectators. It would therefore be very beneficial if a sports lighting fixture could provide a light output pattern which when projected at an angle downward and away from such fixture would result in a light pattern at the playing field surface that would lend itself to a more efficient overlapping configuration and result in less light spill above such field surface.